In a TO-CAN structure where a semiconductor optical modulation element is mounted, a temperature control module may be mounted on a metal stem for the sake of temperature control on the semiconductor optical modulation element.
Patent Literature 1 discloses a technology in which backward light output from a laser diode is passed outside a first plate-like body and received by a photodiode that is supported by a second plate-like body, so that a photodiode is eliminated from the first plate-like body supporting the laser diode to reduce the power consumption of the thermoelectric module.